The present application claims priority to Application No. 102 02 322.0, filed in the Federal Republic of Germany on Jan. 23, 2002, which is expressly incorporated herein in its entirety by reference thereto.
The present invention relates to an internal combustion engine having an exhaust-gas turbocharger and to a method for operating such an internal combustion engine.
Exhaust-gas turbochargers having a variable geometry, a so-called VTG (variable turbine geometry), are described in German Published Patent Application No. 198 57 234 for obtaining an adjusted or adapted pressure build-up of the compressor of the exhaust-gas turbocharger, using a turbine geometry adjusted to the operating point of the internal combustion engine. Such exhaust-gas turbochargers are also used for trucks provided with an engine braking device. During engine braking or a trailing-throttle state, compressed air is supplied to the internal combustion engine by the compressor of the exhaust-gas turbocharger. The precompressed air is compressed again in the cylinder and subsequently discharged through a separate valve at the cylinder head, into the exhaust-system branch, and then travels further to the turbine of the exhaust-gas turbocharger. Since no combustion process takes place in the combustion chambers during engine braking, increased engine-braking action is achieved by a boosted charge.
In this context, the pressures occurring during engine braking can be high enough to damage the turbine of the exhaust-gas turbocharger. In order to rule this out, a bypass, which bypasses the turbine of the exhaust-gas turbocharger when a valve accommodated in the bypass, a so-called waste-gate valve, is in the open position, is provided in the exhaust-system branch. The waste gate valve may be electrically controlled by an electronic control unit. But if the waste gate valve in the bypass malfunctions, e.g., by mechanically jamming, then the pressure increase at the turbine occurring during engine braking can no longer be reduced by releasing gas. As a result, the compressor driven by the turbine continues to generate intense pressure in the intake-system branch. In this context, the pressure can assume values that are greater than the maximum values of the components in the intake-system branch, which causes damage to the component parts. In particular, the pressure increase in the intake-system branch of the internal combustion engine can become so large, that the maximum pressure, for example 2 bar, of an intercooler usually installed downstream from the compressor of the exhaust-gas turbocharger is exceeded, so that the intercooler is damaged. In addition, the turbine can be damaged by the high flow velocities. In particular, vibrations can occur which destroy the turbine blades upon reaching the resonant frequency.
The internal combustion engine of the present invention, which has an exhaust-gas turbocharger, and the method of the present invention for operating such an internal combustion engine, may provide a safety system, which allows a safeguarded engine-braking state, in which an undesirably high pressure increase in the intake-system branch and the exhaust-system branch may not occur, even in response to a malfunction of a valve provided in the bypass.
This safety function may be provided without a large amount of expenditure, since the electric adjustment of or an electric adjustment system for the turbine geometry, in particular the guide-blade adjustment or a guide-blade adjusting system, which is usually already present, may be used. In this context, this safety function may easily be implemented as an additional software module in the control unit already present.
The measures described below may provide further refinements and improvements of the internal combustion engine having an exhaust-gas turbocharger, and of a method for operating such an internal combustion engine.
The FIGURE schematically illustrates an exemplary embodiment of the present invention is represented in the drawing in simplified form and explained in detail in the following description.